This invention relates to a roll-off filter apparatus incorporated in a data transmission system using a time-division multiplexing technique.
FIG. 1 shows the structure of a prior art roll-off filter apparatus. Referring to FIG. 1, a data input terminal 1 is connected to a switch 2. Data buffers 3 and 4 are connected to the data input terminal 1 through the switch 2 and are also connected to a filter 7 through a switch 5. A ramp pattern generator 6 is connected to the filter 7 through the switch 5. A signal output terminal 8 is connected to the filter 7.
Referring also to FIG. 1, a signal having a frequency fi/M (M: an integer) is supplied to each of the data buffers 3, 4 and the ramp pattern generator 6, and signals having frequencies fi and fo are supplied to both of the data buffers 3 and 4.
The operation of the prior art roll-off filter apparatus having the structure shown in FIG. 1 will now be described. Digital data applied as an input to the data input terminal 1 at a data transmission rate of fi bits/sec are alternately supplied to the data buffers 3 and 4 through the switch 2 at the speed fo with the period of fi/M. The data stored in the data buffers 3 and 4 are alternately read out through the switch 5 at the speed fo and with the period fi/M, and, after the ramp signal generated from the ramp pattern generator 6 is added to both ends of each data block corresponding to one frame, the data signal is passed through the filter 7 before it appears as a data output signal from the signal output terminal 8. FIG. 2 shows the waveforms of the input and output signals in the case of the prior art roll-off filter apparatus.
Thus, a burst-like signal having its band limited by the roll-off filter characteristic can be produced from the prior art roll-off filter apparatus too.
However, the prior art roll-off filter apparatus has had such practical problems that the two data buffers 3 and 4 are inevitably required and that an attempt to use a filter, such as, a switched capacitor filter or a digital filter in place of the filter 7 leads to a great amount of power consumption due to the wide signal bandwidth.